In my mind there's basically one solution to this. Make an awesome iconic class a tank, and you'll get a bunch of people playing it.
PLD and WAR are kinda vanilla compared to the other currently available jobs, there's a bigger draw to BLM, DRG, SMN, etc ... and that's aside from the general mindset that more people want to play DPS than tanks.
If they were to add Samurai (with katana) or Soldier (Cloud) as tank jobs, I think it would be less of a problem.